Auto manufacturers continue to request improved fuel economy performance and robustness of future motor oils. For example, the International Lubricant Standardization and Approval Committee (ILSAC) GF-4 specification (final standard was released Jan. 14, 2004 and revised Jun. 1, 2004) requires improvement in fuel economy, high temperature oxidation, high temperature piston deposit, and wear relative to ILSAC GF-3. The ILSAC GF-4 specifies the minimum performance requirements (both engine sequence and bench tests) and chemical and physical properties for those engine oils that vehicle manufacturers deem necessary for satisfactory equipment performance and life.
In addition, ILSAC GF-4 limits the amount of phosphorus to 0.08 wt % in the finished oil. This puts restrictions on the use of zinc dialkyldithiophosphate, a commonly used wear control additive having favorable characteristics as an anti-wear additive. However, a problem has arisen with respect to the use of zinc dialkyldithiophosphate, because phosphorus and sulfur derivatives poison catalyst components of catalytic converters. This is a major concern as effective catalytic converters are needed to reduce pollution and to meet governmental regulations designed to reduce toxic gases such as, for example, hydrocarbons, carbon monoxide and nitrogen oxides, in internal combustion engine exhaust emission. Such catalytic converters generally use a combination of catalytic metals, e.g., platinum or variations, and metal oxides, and are installed in the exhaust streams, e.g., the exhaust pipes of automobiles, to convert the toxic gases to nontoxic gases. As previously mentioned, these catalyst components are poisoned by the phosphorus and sulfur components, or the phosphorus and sulfur decomposition product of the zinc dialkyldithiophosphate; and accordingly, the use of engine oils containing phosphorus and sulfur additives may substantially reduce the life and effectiveness of catalytic converters. Therefore, it would be desirable to reduce the phosphorus and sulfur content in the engine oils so as to maintain the activity and extend the life of the catalytic converter.
Simultaneously balancing ILSAC GF-4 requirements is difficult given that additives typically used to control piston deposits are often detrimental for fuel economy and wear. Through a series of formulation appetite studies, unique combinations of additives have been discovered allowing competing requirements to be satisfied.
Accordingly, as demand for further decrease of the phosphorus content and a limit on the sulfur content of lubricating oils is very high, this reduction cannot be satisfied by the present measures in practice and still meet the severe anti-wear and oxidation-corrosion inhibiting properties, as well as cleanliness (i.e., deposit protection) required of today's engine oils. Thus, it would be desirable to develop lubricating oils, and additives and additive packages therefore, having lower levels of phosphorus and sulfur but which still provide the needed wear, oxidation-corrosion and deposit protection now provided by lubricating oils having, for example, higher levels of zinc dialkyldithiophosphate, but which do not suffer from the disadvantages of the lubricating oils discussed above.
U.S. Pat. No. 6,696,393, issued Feb. 24, 2004 to Boffa, discloses methods and lubricant compositions for reducing wear in internal combustion engines lubricated with a low phosphorus content lubricating oil. The lubricant compositions comprise a synergistic combination of a complex of a molybdenum/nitrogen containing compound and at least one phosphorus-containing compound wherein the total phosphorus employed in the composition is no more than about 0.06 weight percent based on the total weight of the composition.
U.S. Pat. No. 6,562,765, issued May 13, 2004 to Boffa, discloses an engine oil having a base oil and a friction reducing amount of an oil soluble sulfurized or unsulfurized oxymolybdenum complex prepared from reacting, in the presence of a polar promoter, an acidic molybdenum compound and a basic nitrogen compound and a low concentration of a sulfurized oxymolybdenum dialkyldithiocarbamate; employed together to provide at least 450 parts per million of molybdenum and less than 175 parts per million of molybdenum from the dialkyldithiocarbamate, both on the basis of the engine oil.